In an ordinary sewing machine, there are arranged a thread take-up lever mechanism to be driven in an arm by a spindle and a needle bar drive mechanism. A thread take-up lever is partially protruded from a vertical slit in an arm head and is reciprocally driven up and down. The arm is provided with a thread bobbin mount on its upper end side and with a thread tension regulator and a thread tension spring on its front side. A tension dial for the thread tension regulator is provided rear the thread tension regulator. A needle bar protrudes downward of the arm head, and a needle is attached to the lower end portion of the needle bar. A presser foot for pressing a cloth to be sewn and a presser bar for supporting the presser foot can be switched between a bottom position, at which the cloth is pressed by a lifting lever, and an upward retracted position.
In case the sewing is interrupted to exchange the thread bobbin for a needle thread, the presser foot is moved to the retracted position, and the thread tension regulator is brought in a released state. In this state, the thread bobbin is exchanged, and the needle thread fed out from the thread bobbin is guided through a plurality of thread guides to the thread tension regulator. The thread is engaged between a pair of thread tension discs in the released state, with a thread tension spring and further with a thread guard of the thread take-up lever. After this, the needle eye of a needle is threaded with the end portion of the needle thread. When the thread bobbin of the needle thread is to be thus exchanged, the thread has to be engaged with the thread tension regulator, the thread tension spring and the thread guard of the thread take-up lever and has to be passed through the needle eye. These thread engagement and passing operations are troublesome for the user because they are accompanied by the complicated operations, and the user has needed some getting used to doing those operation properly.
Therefore, U.S. Pat. No. 3,749,039 discloses the technique, in which the threading is simplified by mounting the needle thread cassette having the thread bobbin housed therein, removably on the arm. At a transverse center portion of the arm of the sewing machine, there is disposed a cassette mount, in which the needle thread cassette can be removably mounted. In association with the mounting action of the needle thread cassette, moreover, the needle thread fed out from the thread bobbin in said needle thread cassette is automatically engaged with the thread tension regulator, the thread tension spring and the thread guard of the thread take-up lever.
In JP-A-55-81693, on the other hand, there is proposed a cassette type threading device for a sewing machine. This cassette type threading device is provided with a thread take-up lever travel region of an arm, a cassette mount disposed on the right side of the region, and a cover member for opening/closing the cassette mount. The needle thread cassette having the thread bobbin housed therein is removably mounted in that cover member. A thread tension regulator and a thread take-up spring protrude into the cassette mount. The cover member is opened by turning it 90 degrees forward and is set with the needle thread cassette. By doing the action to close the cover member with the thread take-up lever being at the lowermost position, the needle thread is automatically engaged with the thread tension regulator and the thread take-up spring. When the thread take-up lever is then raised, the needle thread is automatically engaged with the thread guard of the thread take-up lever.
As disclosed in JP-A-5-293284, for example, there is practiced a sewing machine, which is constructed to have an automatic threading mechanism at the arm of the sewing machine so that the end of the needle thread is passed through the eye of a needle by moving the control lever of the mechanism downward by a predetermined distance.